1. Field of the Invention
The present invention relates to an oscillation device for oscillating a terahertz wave.
2. Description of the Related Art
For active layers of oscillation devices each of which oscillates a terahertz wave (electromagnetic wave having a frequency in a range of 30 GHz to 30 THz in this specification), a quantum cascade laser and a resonant tunneling diode (hereinafter referred to as “RTD”) are mainly used. These oscillation devices realize electromagnetic wave oscillation based on the intersubband transition of electrons in a semiconductor quantum well structure.
In particular, it is expected that an oscillation device using a double-barrier type RTD as the active layer is able to oscillate a terahertz wave in a frequency domain in the vicinity of 1 terahertz at room temperature. The double-barrier type RTD is formed of an InGaAs quantum well layer and InAlAs tunnel barrier layers, which are epitaxially grown on an InP substrate. In the region of a differential negative resistance which appears in voltage-current (V-I) characteristics, the RTD oscillates an electromagnetic wave by stimulated emission.
In order to use the oscillation device described above in the field of communication technology and/or imaging technology, the oscillation device is required not only to be small but also to be able to modulate the frequency of an oscillating terahertz wave.
An oscillation device in which the frequency of an oscillating terahertz wave is changed by changing a drive voltage in the above differential negative resistance region is disclosed in a non-patent document: ELECTRONICS LETTERS Vol. 41, No. 15, 872. In this case, a center frequency is 470 GHz, and a variable width of the frequency is 18 GHz. The above drive voltage functions, of course, as current injection into an RTD, and if the voltage is changed in order to change the frequency, for example, the power of the oscillating terahertz wave is also changed; hence, an improvement is required in view of controllability.
Accordingly, an oscillation device which changes the frequency of an oscillating terahertz wave is disclosed in Japanese Patent Laid-Open No. 8-125215 in which the intensity of light radiating on an RTD from the outside is changed, and the differential negative resistance and parasitic elements of the RTD are changed.
In the structure disclosed in Japanese Patent Laid-Open No. 8-125215, since light radiates on the RTD from the outside, a window structure for light incidence must be provided. This window structure is formed by making a hole in an electrode provided on an upper portion of an active layer. According to this window structure, since the contact area between the electrode and a layer provided thereunder is decreased, series resistance which is one of the above parasitic elements is increased. As a result, since the RC time constant is increased, and the upper limit of a cut-off frequency is decreased, the frequency band of an oscillating terahertz wave is narrowed.